Heat transfer petroleum recovery process



Jan. 8, 1963 D. c. BQND ETAL 3,072,191

HEAT TRANSFER PETROLEUM RECOVERY PROCESS Filed April 10, 1961 4 28 f 7 7a g k E 7 A TTORNE Y United States Patent ()1 3,t 72,l )1 HEAT TRANSFERPETROLEUM REQQVERY PROESS Donald C. Bond and Leo d. Giirien, flrystalLake, Iil., assignors to The Pure Oil Company, Chicago, lilL, acorporation of Uhio Fiied Apr. 1%, W61, Ser. No. 1%,6998 5 Claims. (Cl.166-39) This invention relates to a novel in-situ combustion process forincreasing the rate of production from petroleum wells. The method ofthis invention is applicable to increasing the production of petroleumfrom formations subject to plugging by waxy, petroliferous depositswhich form in the formation rock surrounding the wellbore.

In certain oil reservoirs, non-viscous crude oils are produced whichcontain large amounts of wax. Under virgin reservoir conditions the waxis contained in solution in the reservoir fluid. When the reservoir isproduced through a well, wax deposits in the formation rock surroundingthe well, and eventually plugs the formation. The flow capacity of suchformations is reduced to a very low value. Fracturing the well throughthe region where the wax has formed temporarily alleviates thesituation, but wax deposition continues in the rock surrounding thefractures, and plugging again occurs.

It has been proposed to clear such wells by disposing heaters in thewell-bore adjacent to the plugged formation, the heaters being suppliedwith air and a gaseous fuel. Such methods have not been verysatisfactory because of the inability of the heat produced tosatisfactorily penetrate the formation countercurrent to the directionof petroleum flow and melt the waxy deposits. The amount of heatproduced by the burners must be closely regulated or the temperature inthe well-bore may rise to a point deleterious to the down-holeequipment. Over a period of time, even moderately high temperaturesresult in destruction of down-hole equipment. It has further beenproposed to institute in-situ combustion in paraffin-plugged formations.The formation wall is heated to a high temperature by means of adown-hole Well-bore heater, and then a free-oXygen-containing gas isinjected into the formation to maintain combustion of the combustibleformation constituents. While such methods are effective for the removalof wax deposits from the formation surrounding the Well-bore, the flowof oil to the well-bore is necessarily seriously disrupted, and it isfrequently difficult to restore the treated well to normal production,Moreover, after the well-bore is returned to production and thetemperature drops by conductive heat transfer to about normal formationtemperatures, wax again deposits and production rnust again beinterrupted so that the well may be treated.

In some instances, where waxy oils are produced from apetroleum-containing stratum, it is found that a subadjacent orsuper-adjacent stratum exists which is permeable, but is separated fromthe petroleum-producing stratum by an impermeable layer of rock. Wherethe permeable super-adjacent or sub-adjacent formation containsquantities of non-commercial combustible materials, it has been proposedto initiate in-situ combustion in such strata, and propagate aflame-front through the stratum between injection and outlet wells. Inthis manner, heat may be transferred through the impermeable layers tothe petroleum producing formation, so that the temperature of thepetroleum is raised and the viscosity reduced. While such treatmentshave been helpful in some circumstances, they are not well adapted topreventing wax deposits adjacent to the producing well because theflamefront in the adjacent stratum remains in the zone of thepetroleum-producing well-bore only for a short time, and

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2 is propagated outward through the stratum so that heat transferredthrough the impermeable layer is for the most part not effective formelting waxy deposits adjacent to the producing well-bore. It is in thezone immediately surrounding the producing well that reduction ofpermeability due to wax deposits is critical, since it is here that therate of fiow of oil is necessarily the highest. Moreover, that petroleumwhich is effectively heated cools as it passes through the formationrock from the location at which the flame-front resides.

It is an object of this invention to provide an improved method ofin-situ combustion by which heat can be liberated in a super-adjacent orsub-adjacent fuel-containing stratum and transferred through animpermeable rock layer to a producing formation surrounding anoil-producing Well, whereby viscous petroleum deposits can beeffectively melted and the rate of petroleum production increased.

This invention can best be described with reference to the drawing,which shows in section a petroleum reservoir which is penetrated by awell-bore. The reservoir comprises a petroleum-producing stratum 1t andimpermeable layer 12, and a permeable stratum 14 which containsnon-commercial, 600-B.t.u.-per-cubic-foot natural gas. To be susceptibleto treatment in accordance with this invention, the reservoir must havean impermeable layer with a thickness within the range of about 5 toabout 50 feet and a permeability which is preferably less than about 0.01 rnillidarcy. The porous, gas-containing stratum preferably has athickness in the range of about 10 to feet, and a permeability which ispreferably in excess of about 5 millidarcies. The well casing 16 isperforated at 118 to communicate with the petroleum-producing zone 10.Oil-producing tubing string 29 extends through the well-bore to a pointadjacent to perforations 18. A packer 22 is set at the impermeable layer12 to isolate the portions of the wellbore lying above and below theimpermeable layer. The casing is perforated at 26, adjacent to the lowerextremity of the gas-containing stratum 14, and is also perforated at 28adjacent to the upper extremity of the gas-containing stratum 14. Inthis manner, communication is established between the Well-bore and thelower portion of the gas stratum through perforations 26, and betweenthe well-bore and the upper portion of the gas stratum throughperforations 28. A second tubing string 30 is also disposed within thewell-bore. This tubing string terminates at burner 32, which ispositioned adjacent to perforations 25. A second packer 34 is placedwithin the well-bore, between the perforations 2s and 2%, to isolate awell-bore zone 36 which lies between packers 22 and 34, and an upperwell-bore zone 38 which lies above packer 34.

Combustion in the gas-producing stratum 14 may be instituted in aconventional manner, by supplying a combustible mixture of gas and airto the burner 32 by way of tubing string 3t}. While the temperatureswithin the well-bore during the period in which in-situ combustion isinitiated will be undesirably high, these temperatures need bemaintained for only a relatively short period of time. After combustionhas been initiated, by means of the burner 32 as described, oralternatively by other techniqu-es well known in the in-situ-combustionart, the flow of gaseous fuel to tubing string 39 is terminated, andair, or other combustion-supporting gas, is pumped down tubing string3d, through burner 32, and into the lower portion of gas-containingstratum 14. Combustion products and unreacted nitrogen from the injectedair are exhausted to well-bore zone 38 through the perforations 28, andthen flow upward through the well-bore in heatexchange relationship withtubing strings 20 and 30. Heat transfer from the partially cooled finegases to the produced oil in tubing string 20 eliminates the waxydeposits Within tubing string 20, and heat transfer to air pumpeddownward in tubing string 30 serves to preheat the air before injectioninto the gas-containing formation 14.

Natural gas in stratum 14 migrates towards the lowpressure zone at thewell-bore, and would seek an outlet through the perforations 28 but forthe injection of combustion-supporting air into the stratum throughperforations 26. In this manner, a combustion flame-front can bemaintained in the formation within a short radial distance outward fromthe well-bore, without causing the flame-front to be propagated throughthe formation. The flow of gaseous fuel is constantly towards thelocation of the flame-front, and the front itself is static. Outlet forthe combustion products is provided as aforedescribed.

It is evident that by the method of this invention the zone of burningcan be maintained within relatively close proximity of the well-bore,preferably within about 30 feet of the Well-bore. Heat transferreddownward through the impermeable layer 12 is applied to the producingformation at the zone of critical pressure-drop, Where viscous petroleumdeposits are a severe problem. The waxy deposits are melted and movealong with the produced petroleum to the well-bore, and then flow to thesurface of the earth through tubing string 20. It is evident thatproduction of oil can be continued uninterrupted throughout the durationof the combustion process, which is preferably substantially coextensivewith the productive life of the petroleum reservoir.

It also is evident that it is necessary to rniantain the rate andpressure of oxygen-containing gas injection within reasonable limits.Excessive injection rate will result in an advance of the flame-frontaway from the well-bore, so that heat will be produced at a pointundesirably remote from the wellbore, and the amount of heat thenavailable to melt waxes deposited in the producing formation in the zoneimmediately around the Well-bore will be reduced. On the other hand, toolow an injection rate may result in back-burning to the well-bore, withthe accompanying danger of explosion or injury to down-hole equipment byexcessive well-bore temperatures. While the correct rate and pressure ofinjection will vary from reservoir to reservoir, depending upon the sizeof the natural-gas-containing stratum, the quantity, pressure, andcalorific value of the gas contained within the reservoir, as well asthe number of exhaust perforations 28 provided above the packer 34, itis possible to' provide the correct injection rate from an analysis ofthe flue gases produced through the well-bore annulus. A large excess ofunreacted oxygen in the flue gases indicate that excessive quantities ofair are being injected, and the flame-front is progressing outward awayfrom the well-bore. On the other hand, the production of unburnednatural gas along with the flue gases indicates that insufiicient air isbeing injected and that the fire in the formation is limited to theimmediate vicinity of the perforations 26. The absence of large amountsof unreacted oxygen or unburned natural gas in the combustion productsindicates a proper air injection rate. The injection pressure should beadjusted to maintain this rate throughout the life of the reservoir. Ingeneral, as the pressure of the natural gas declines, the injectionpressure may be reduced.

As a specific example of the method of this invention, apetroleum-producing reservoir in which production has declined due tothe accumulation of wax deposits in the formation adjacent to thewell-bore is treated by in-situ combustion. Above the oil-producingstratum lays an impermeable layer having a thickness which varies fromto 25 feet. Above this impermeable layer lays a permeable stratum ofMississippian rock containing noncommercial 600-B.t.u.-per-cubic-footnatural gas. The initial gas pressure is 1000 pounds per square foot. A

packer is set adjacent to the impermeable layer to separate and isolatethe portions of the Well-bore lying above and below this layer. A tubingstring terminating in a gas burner is lowered to a point a shortdistance above the impermeable layer, and a packer is set above theburner to separate an injection well-bore zone around the burner, and anexhaust well-bore zone above the burner and second packer. A combustiblemixture of gas and air is pumped down the tubing string to the burnerand combustion in the burner is instituted. Combustion is maintained fora period of 15 hours, during which time the gas-containing formationrock surrounding the wellbore is heated to ignition temperatures;thereafter, the passage of gas to the burner is terminated, and air isinjected through the tubing string leading to the burner at a pressureof 1275 psi. and a rate of 500 cubic feet per hour. Combustion ismaintained in the formation surrounding the well-bore, and combustionproducts reenter the well-bore above the second packer. The combustionproducts flowing from the well-bore to the atmosphere are analyzed, andfound to contain free oxygen in the amount of about 20% of the injectedoxygen. The injection pressure is reduced to 1200 pounds per squareinch, and the injection rate thereupon drops to 400 cubic feet per hour.After the lapse of two hours, the flue gases are again analyzed andfound to have a free oxygen content of 3%. The injection pressure andrate are maintained, and petroleum is produced at gradually increasingrates from the underlying petroleum-containing stratum.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows.

We claim:

1. In the recovery of oil through a well-bore penetrating a subterraneanreservoir, comprising an oil producing stratus, acombustible-gas-producing stratus vertically spaced therefrom, and asubstantially impermeable stratum lying between said producing strata,wherein viscous deposits of petroliferous material form in and plug theoil-producing stratum around said well-bore, the improvement comprisingisolating zones of said well-bore adjacent to said oil-producing andgas-producing strata from each other, initiating combustion of gas insaid gas stratum, maintaining combustion in the gas stratum near to saidwell-bore by the injection of a free-oxygencontaining gas from saidwell-bore into a portion of said gas stratum near one vertical extremitythereof, venting combustion products to said well-bore from a portion ofsaid gas stratum near the other vertical extremity thereof and producingoil from said oil producing stratum through said well-bore.

2. The method in accordance with claim 1 in which said oxygen-containinggas is air.

3. The method in accordance with claim 2 in which the quantity of airinjected is adjusted to produce combustion products substantially freeof excess unreacted oxygen and combustible gas.

4. The method in accordance with claim 1 including the steps ofperforating the well casing adjacent to said portions of thegas-producing stratum, and placing a packer in the well-bore between thesaid portions to isolate two well-bore zones, one zone being adjacent toeach of the said portions. 7

5. The method in accordance with claim 3 wherein the well-bore containsan oil-producing tubing string and air-injection tubing string, andcombustion products are exhausted through the well-bore externally ofthe tubing strings in heat-exchange relationship therewith.

Merriam et a1. Feb. 5, 1952 Tadema Feb. 24, 1959

1. IN THE RECOVERY OF OIL THROUGH A WELL-BORE PENETRATING A SUBTERRANEANRESERVOIR, COMPRISING AN OIL PRODUCING STRATUS, ACOMBUSTIBLE-GAS-PRODUCING STRATUS VERTICALLY SPACED THEREFROM, AND ASUBSTANTIALLY IMPERMEABLE STRATUM LYING BETWEEN SAID PRODUCING STRATA,WHEREIN VIS-